Pneumatic springs, commonly referred to as air springs, have been used in motor vehicles for a number of years to provide cushioning between movable parts in the vehicle. Air springs absorb shock loads impressed on the vehicle axles by the wheels striking an object in the road or falling into a depression. An air spring typically consists of a flexible rubber sleeve also known as a bellows or a bladder. The sleeve contains a supply of compressed gas and has one or more pistons located therein or thereunder. An end cap closes off the upper region of the sleeve. During use, the piston moves axially toward and away from the end cap so as to alternatingly compress and expand the internal volume within the sleeve. In this manner, the air spring acts as an energy absorbing means for the vehicle, with operating characteristics determined by the spring rate of the air spring.
It is commonly known that vibration isolation, occupant ride quality, and cargo protection can be improved by reducing a vehicle suspension's dynamic spring rate. As is known in the art, one method of reducing the spring rate of an air spring is to increase the interior volume of the sleeve. Although increasing the internal volume of the sleeve may reduce the spring rate of an air spring, increasing the interior volume of the air spring is not suitable for many applications. For instance, in applications where the space for mounting the air spring is limited, increasing the interior volume of the air spring may prevent proper installation of the air spring, or worse, may prevent the use of the specifically designed air spring.
Further known in the art, a reduction of the spring rate of an air spring may be achieved by the use of alternative gases within the interior volume of the air spring. For example, the ratio of heat capacities, sometimes referred to as the ratio of specific heats and denoted by the Greek letter “gamma,” varies with the molecular size of the gas. Accordingly, selecting a gas with a lower gamma coefficient than air will provide an air spring with a reduced spring rate without increasing its interior volume. However, these alternative gases contain more complicated molecules, such as sulfur hexafluoride with a gamma of 1.09, that often pose problems, such as high costs, lack of availability, toxicity, and system compatibility.
Therefore, there is a need in the heavy truck industry for a method and apparatus for reducing the spring rate of an air spring that is incorporated in a suspension of a heavy truck.